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Fuel and chemical co-production from tree crops
Biomass 9 (1986) 49-66
Fuel and Chemical Co-Production From Tree Crops Michael Seibert*, Gregory Williamst, Gray Folger* and Thomas Milne* *Solar Energy Research Institute,$ 1617 Cole Boulevard, Golden, CO 80401, USA ?International Tree Crops Institute, USA, Inc., Route 1, Gravel Switch, KY 40328, USA (Received: 13 July, 1985)
A BSTRACT A concept for the sustainable production of fuels and chemicals from fermentable and oleaginous substrates produced on an annual basis by the reproductive organs (pods, fruits, nuts, berries, etc.) of'tree crops' is presented. The advantages of tree-crop systems include suitability for use on marginal land, potential productivity equivalent to row crops, minimal maintenance and energy input requirements, environmental compatibility, and the possibility of coproduct production. Disadvantages are possible high establishment costs, a long growth period until production commences, and difficulties in harvesting or storage; however, these provide opportunities for potential research impact. Honeylocust, mesquite, persimmon, osage orange, and Chinese tallow are examined as potential US tree-crop species. Other species, including breadfruit and African oil palm, are suggested as tree-crop candidates for the tropical developing world. Fermentation or extraction of treecrop organs and the economics of tree-crop systems are also discussed. Currently, the greatest area of uncertainty lies in the actual pod or fruit yields that can be expected from large tree farms under real life conditions. However, ballpark ethanol yield estimates of 650 to 34701itresha -I (69-371gallonsacre -l) and oil yields up to 3900 kg ha- l (4227 litres ha- i) justify further consideration of treecrop systems. Key words: Tree crops, alcohol, oil, biomass, marginal land, fuels and chemicals. :~Operated by the Midwest Research Institute for the US Department of Energy under contract No. DE-AC02-83CH 10093. 49 Biomass 0144-4565/86/S03.50 - © Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain
50
M. Seibert, G. Williams, G. Folger, T. Milne
INTRODUCTION About 50 years ago Smith 1 introduced a new agricultural concept which he termed 'tree crops'. The notion was to develop a sustainable foodproduction system based upon the use of previously uncommercialized trees grown for an annual crop of pods, fruits, nuts, or berries. The use of tree crops for fuel and chemical production, an extrapolation of Smith's ideas, is examined here. It should be emphasized at this point that we use the term 'tree crops' in the historical sense, in contrast to 'crops of trees' utilized in silviculture energy plantations where wood or energy-rich components found in the wood are the primary products. This paper considers trees that produce large amounts of sugars, other carbohydrates, or oleaginous materials in specialized organs on a perennial basis. These materials can be viewed as a source of fermentable substrates for ethanol production, oils for diesel fuel substitutes, or raw materials for the chemical industry
A D V A N T A G E S OF TREE CROPS The recent resurgence of interest in fuel alcohol production from fermentation of traditional agricultural crops, particularly grains, has heightened concern for the ecological and policy implications of a perceived 'food vs fuel' conflict 2 and for the conversion of additional land to grain production. Alcohol obtained from tree-crop sources circumvents this potential conflict because the trees can be grown on marginal land, including that with poor soil, steep terrain, and semi-arid conditions unsuitable for row crops. Oil-producing tree crops exhibit similar advantages. As will be documented later, the potential annual productivity of certain species is equivalent to or higher than row crops. In addition, tree cultivation may require minimal maintenance and energy input; is conducive to soil amelioration, erosion prevention, water retention, and wind protection; provides an environment suitable for wildlife and other multiple uses; and can produce many coproducts such as lumber, firewood, protein, specialized chemicals, honey, and fiber, to name a few. SPECTRUM OF P O T E N T I A L C A R B O H Y D R A T E - P R O D U C I N G WOODY SPECIES Table 1 presents a list of potential, woody perennials which have been considered as candidate tree-crop species in the US. 3, All grow over
Fuel and chemical co-production from tree crops
51
large portions of the country (though Eastern species are emphasized), and all produce large numbers of reproductive organs that contain large quantities of sugars and/or other carbohydrates. For convenience the table is divided qualitatively into three feasibility categories based on potential equivalent alcohol yields ha-land tree maintenance requirements necessary for high yields. Most of the candidates can be eliminated based on low potential alcohol production yields and high maintenance requirements. More information on the 'high feasibility' species of Table 1 is given in Table 2. Ethanol yields for corn are noted for comparison. The estimated ethanol yields in Table 2 (except mesquite) are based on literature values for reproductive organ yields and composition (% sugars and other carbohydrates), theoretical conversion of 1 mole of
TABLE 1 Feasibility Ranking of Selected Woody Perennials for Ethanol Production a
High feasibility ( high yields, b low maintenance")
Minimal feasibility (high yields, high maintenance)
Blackberry (4) Blueberry (4) Elderberry (5, 6) Hawthorn (7, 8) Highbush cranberry (6,7,9) Honeylocust (10, 11) Jujube (12) Mesquite (13) Mulberry (14, 15) Oriental persimmon (14,16) Persimmon (1, 14, 17) Raspberry (4, 14) Sawtooth oak (7, 18) Serviceberry (6, 7, 19)
Apple (4, 20) Cranberry (4) Grape (4, 9) Peach (4, 14) Pear (4, 21) Plum (4, 14) Sour cherry 4, 22) Sweet cherry (4, 22)
Unfeasible (low yields) American beech (8, 23) Chinese chestnut (18, 23, 24) Currant (4, 6, 14) Dogwood (7, 8) Eastern black walnut (23,25) Gooseberry (4, 6, 14) Hazel (7, 18) Holly (8, 26) Various oak species (7, 18) Papaw (27) Pecan (18, 23) Persian walnut ( 18, 23) Shagbark hickory (23, 26) Wild plum (8)
a Numbers in parentheses indicate references. b High yields are defined as estimated potential ethanol yields of >/468 liters ha-~ (50 gallons acre-1) though maximum values in almost all cases were over 935 liters ha(100 gallons acre-1). Low yields are defined as a maximum potential of ~<468 liters ha-1. c Maintenance was judged on requirements for spraying (insect and disease control) and pruning to maintain high yields.
Small tree/pome
Tree/acorns
281 - 1216
c. 655 (at 10 yr old)
Precocious
Arid land species Subject to animal damage; fruit stores poorly Difficult to establish; subject to late frost damage; hardiness problems; fruit stores poorly Drought resistant and tolerates poor site conditions; difficult to establish but are hardier and flower later than D. kaki; subject to wilt disease in the south; fruit stores poorly Precocious, annual bearing
Drought resistant and tolerates poor site conditions; tends to alternate bear; susceptible to mimosa webworm
Precocious
Requires acid soil; subject to animal damage; fruit stores poorly Precocious; fruit stores poorly
Subject to animal damage; fruit stores poorly
Comments
"See Table 1 for references. b For comparison, corn at 100 bu acre- t would yield 2067 liters ha- ~of 100% ethanol based on an 85% fermentation yield. To convert liters ha- Lto gallons acre t, divide by 9.353.
Sawtooth oaks (Quercus acutissima Carr.) Serviceberry (A melanchier sp. Med.)
1309-3367
1496-3741
Tree/pod
Tree/berry
c. 1029 561-2619
Small tree/pome Small tree/drupe
c. 1403 882-1510 c. 935 748-5986
374-1029
Small tree/drupe
Tree/drupe Tree/pod Tree/drupe aggregate Tree/berry
286-1496
Shrub/berry
Jujube (Ziziphusjujuba Mill.) Mesquite (Prosopissp.) Mulberry (Morus sp. L.) Oriental persimmon (Diospyros kaki L.) Persimmon (Diospyros virginiana L.)
1964 maximum
Vine or shrub/drupe
Blackberry, raspberry (Rubus sp. L) Blueberry ( Vaccinium corymbosum L.) Elderberry (Sambucus canaensis L.) Hawthorn ( Crataegus sp. L) Highbush cranberry ( Viburnum trilobum Marsh.) Honeylocust ( Gleditsia triancanthos L.)
Estimated ethanol h yield ( litres ha - ~yr t )
Plant/organ type
Name
TABLE 2 E s t i m a t e d E t h a n o l Yields from, Selected W o o d y Perennials"
e~
L~ t'~
M. Seibert, G. Williams, G. Folger, T. Milne
53
sugar (glucose or fructose) to 2 moles of ethanol, and fermentation yields of 85%. These specific values used to convert sucrose, starch, and invert sugar to ethanol were 0.579, 0.609, and 0.551 litres kg -1, respectively. For mesquite, osage orange, and breadfruit we used the same assumptions except that 0.462 litres kg-1 was used to convert carbohydrate to ethanol based on the assumption that carbohydrate in these species is similar to that in corn. We should emphasize that the ethanol yield values given should be taken as best current estimates. In many cases the yields are based on production results from small numbers of trees for which the growth conditions were uncontrolled or undocumented. With this important caveat we have chosen honeylocust, mesquite, and persimmon from Table 2 as three species warranting further investigation because they tolerate relatively poor site conditions.
CARBOHYDRATE SPECIES WARRANTING ADDITIONAL INVESTIGATION In this section we shall give relevant background information on each of the species identified in the last section as warranting further investigation.
Honeylocust (Gleditsia triacanthos L.) Honeylocust is native to the central United States and can be found east of the Mississippi from Alabama and Georgia to New England. Superior thornless varieties ('Millwood' and 'Calhoun') are subject to few pests and diseases (at least in the wild). The mimosa webworm, however, can inflict serious damage. Deer have killed many thornless trees planted for yield trials, but hanging bars of deodorant soap in the trees seems to keep them away. Honeylocust can tolerate poor growing conditions with yearly precipitation levels as low as 0.5 m. The trees have an open foliage canopy, which is advantageous for grass or (nitrogen-fixing) legume production underneath. They are either predominantly pistillate (female, pod producing) or staminate (male, pollen producing) and can be propagated vegetatively. For good pod production some staminate trees should be available. Planting densities of 86 to 111 trees ha- ~ are probably optimal, and fertilization is required especially if the land is also to be used for grazing of livestock. Significant production of long flat, dark brown pods starts when vegetatively propagated trees are about 6 years old. Trees propagated in this manner bear at a younger age than seedlings. Nine-year-old 'Millwood' trees (99ha -~) have reportedly
54
M. Seibert, G. Williams, G. Folger, T. Milne
produced up to 8068 dry kg of pods ha-l, though they tend to alternately bearY Pruning may minimize this problem. Pods could be harvested mechanically using tree-shaking machines or chemical pod abscission agents and the pods raked up with conventional haying equipmentfl9 Total pod sugar contents of up to 42.3% and protein contents up to 15.4% have been recorded (dry weight). Cattle feeding trials have demonstrated that the pods can be substituted for oats on a weight basis. The tree itself is not a particularly fast grower but can attain a height of 21 to 24 m at maturity (120 years). Honeylocust has been used extensively in Midwestern shelter belts. Further information can be found in reports by Scanlon, 28 Williams and Merwin, 29 and the USDA Forest Service.3° Large-scale planting trials are underway. 31
Mesquite (Prosopis sp.) Mesquite is a nitrogen-fixing, cold-sensitive, salt-tolerant shrub or tree found in marginal, semi-arid land in the southwestern United States. The wood commands premium prices as both lumber and firewood. Currently it is also being converted to charcoal. However, prior to 1978 the only US government-related effort with mesquite was an eradication program to eliminate it from range land. The tree shows some susceptibility to psyllid insect and mistletoe damage. Based on 6% moisture content, pod yields of 4000 kg ha-~ year-~ should be possible under range land conditions; and at 200 trees ha- 1,where water is not limiting, 10 000 kg of pods ha-~ year-1 might be expected. Harvesting could be done as with honeylocust. The pods contain from 13% to 36% sucrose and from 11% to 17% protein (at 2% water content). High yielding, thornless trees should be selected and propagated. In this respect, some progress has been made initiating shoots by plant tissue culture techniques (E Felker, pers. commun.). More detailed information is available from Felker et al. ~L~
Persimmon (Diospyros virginiana) The common persimmon is adaptable to poor soil conditions but requires more precipitation than honeylocust. It is found in the southeastern United States from southern Florida to southern Connecticut and west to Texas and Kansas. It can be propagated vegetatively by grafting on to established rootstock. Trees should be planted at a density of about 200 trees ha- 1. A few male trees should be included for pollen, but cultivars that set nearly full-sized seedless fruit parthenocarpically are known. This characteristic may offer a distinct advantage
Fuel and chemical co-production from tree crops
55
for processing. Trees reach a height of 15 to 32 m (shorter under dry conditions) and are important commercially for their wood. Several insect species can attack persimmon but do not cause much damage. However, the persimmon borer (Sannina uroceriformia)could be a very serious pest in large-scale plantings, and a fungus (Cephalosporium diospyri) disease has been known to kill many trees in the southern states. Deer browsing has been suggested as a potential problem. In reality it has not been a concern during yield trials in Kentucky; however, opossums and other wildlife may be a problem. Some tree selection has been accomplished over the past century, but emphasis has been on fruit quality for human consumption. Some 10-year-old trees will produce fruit, but maximum production occurs at age 25 to 50 years. Grafted trees begin fruiting after 3 to 4 years. The tree does not tend to alternately bear to a great extent.The fruit ripens over a relatively long period (about a m o n t h ) a n d does not store well. However, some cultivars produce fruit that can be dried without molding. More information can be found in References 29, 30, 32.
OTHER CANDIDATE CARBOHYDRATE-PRODUCING SPECIES NOT PREVIOUSLY CONSIDERED Osage orange (Maclura pomifera) is native to Arkansas, Texas, and Oklahoma but grows as far north as New England. It produces an unpalatable, multiple fruit that is a globose, yellowish-green structure weighing about 0.45 kg, 80% of which is water. 33 The tree starts to bear at 10 years of age, lives 150 years or more, and can reach a height of 9-12 m. It can be propagated vegetatively and is one of the most insectand disease-resistant tree species in North America. The fruits fall to the ground after the first hard frost and store fairly well for several months at cool temperatures. One of us (T.M.) has observed a yield of 300 fruits produced per female tree in Missouri, which is less than reported values of up to 450 kg of fruit per tree? 4 The fruit contains about 15% sugars and 7% other carbohydrates on a dry weight basis. The tree can be planted at very high density, but if planting densities of 100 trees ha(10 m centers) are used, 1073 liters ha -~ year -~ of ethanol could be obtained using previous assumptions. Though the estimated yield is rather low, there has been no selection of osage orange for fermentable fruit carbohydrates, and other coproducts representing 78% of the fruit dry weight are potentially valuable. For example, the fruit contains proteolytic enzymes that have potentially important economic uses and many secondary metabolites, oils (about 18.5% on a dry weight basis),
56
M. Seibert, G. Williams, G. Folger, T, Milne
and resins 35 which may also be of use. The wood is very hard, resists rotting, and is still used for fence posts. Probably the species' most important use in the past has been as 'living fences'. Tropical species provide a wealth of genetic material and should not be overlooked. Possible US applications may be suitable for Puerto Rico or Hawaii, but certainly there is much potential in the developing world where fuel requirements are great, oil imports expensive, and labor is abundant and inexpensive. The positive Brazilian experience using sugar cane as the raw material for fermentation to ethanol is well known, but breadfruit (Artocarpus communis J. R. and G. Forst) may provide another opportunity. 36 The tree grows well in tropical lowlands that receive adequate rainfall. In the Caribbean the only serious disease problem is caused by Rosellinia sp., which is associated with poor agricultural practices. Insect problems seem to be minor. Fungus is a much more serious problem in the South Pacific. The fruit weighs about 0.9 kg (wet weight) on average and contains from 68% to 80% water. On a dry basis, it contains between 72% and 86% carbohydrate. Production yields of 6993 to 30 033 wet kg ha- 1year- 1 depending on water availability have been reported. This would give 655-2850 liters of ethanol ha-l year ~ using the previous criteria. The tree is already grown in the Caribbean as a source of food (though not on plantations as might be necessary for use as a fuel crop). Finally, the species can be propagated from root suckers.
FERMENTATION Avgerinos and Wang 37 have fermented both honeylocust and mesquite pods using a mixed culture of two thermophilic bacteria. In the case of mesquite the organism utilized up to 83% of the total fermentable carbohydrates (which represents 1.5 to 1.8 times the sugar contents mentioned earlier in this paper) with the production of ethanol at 80% of the theoretical maximum yield. Preliminary fermentation results with honeylocust were disappointing due perhaps to a substance in the pods that inhibited these particular bacteria. Ogden 3~ has also reported limited success in attempting to ferment honeylocust pods.
OIL-PRODUCING TREE CROPS Oil seed crops have considerable potential as a renewable source of petroleum substitutes. Tests indicate that vegetable oils may be used
Fuel and chemical co-production from tree crops
57
directly as a diesel fuel substitute in indirect injection but not in direct injection diesel engines (the most c o m m o n type). 39 However, these oils can be modified to substitute for petroleum-based products. For example, they can be trans-esterified to function in direct injection diesel engines, or they can be converted to gasoline by exposure to a zeolite catalyst at high temperature and moderate pressure. 4° In this section we will discuss two promising tree species that could have significant impact on the fuel and chemical industries. Both are currently being produced commercially outside of the US, but not for fuel use. For comparison, Table 3 shows yields, world production figures, and prices of various vegetable oils. Chinese tallow
The Chinese tallow tree, Sapium sebiferum (L.) Roxb., a Euphorb, is a native of China, where it has been cultivated for as long as a thousand years. In the US, it grows on marginal lands along most of the coastal region from North Carolina to south Texas and in southern California. It is a deciduous, subtropical species that is small (10-14 m) and shortlived (40 to 50 years). The tree grows in low, wet areas, thrives in poorly drained soil, and is salt tolerant. In addition, its deep root system gives it considerable drought tolerance. Insects and disease have not been problems perhaps due to the high levels of phenolics in the leaves and sap. However, defoliation due to leaf cutting ants (Atta texana) and the yellow-striped army worm (Spodoptera ornithogallis)has been observed in south Texas. Neither is seen as a major threat, a5 Fruits develop in clusters of three- or four-celled capsules which contain a single pea-sized white seed. A white greasy mass, or vegetable tallow, covers the seed, and the seed kernel contains a liquid oil called stillingia oil. Both are triglycerides, the former containing mostly saturated fatty acids and the latter, unsaturated fatty acids. The tallow is very similar in properties to cocoa butter. It is edible and has been used in candle and soap making. The seed oil is similar to drying oils. It has been used as an illuminating or lubricating oil, in paints and finishes, and in cosmetics. Both the tallow and the oil are currently used in the Chinese chemical industry (H. W. Scheld, pers. commun.). The composition of the seed is highly variable with tallow content ranging from 11% to 32% and the oil content from 15% to 30% by weight. The seed also contains a thiamine-rich but lysineand methionine-deficient protein ( - 1 1 % ) suitable as a livestock feed supplement or for use in baking flour. Seed yields of 11 205 kg ha- t at 395 trees ha- ~ have been reported and estimates of up to 5043 kg of tallow plus oil h a - 1 have been suggested. 43,46
58
M. Seibert, G. Williams, G. Folger, T. Milne
Chinese tallow is also attractive in the number of products that the tree provides. Besides the seed products discussed above, the tree can be used as a fuelwood crop since it is adaptable to close spacing and direct seeding, it grows rapidly for the first 6 years (10-15 dry tonnes ha -~ year -1 in densely planted stands; dry wood from 2-year-old trees averages 4225 kcal kg-~), and coppices. 45 Older trees partition most of their photosynthate into seed production, though seedset can commence as early as the third season of growth. On the other hand, the tree is spread rapidly by birds ingesting and disseminating seeds and thus could become a 'weed' problem. The leaves produce a black dye used by the Chinese for silk and could be a commercial source of industrial tannins. About 112 kg of honey, with a wholesale value of S123 has also been reported per hectare. 43
African oil palm Hodge 4t has reported that the African oil palm (Elaeis guineensis Jacq.) is the most efficient oil-producing palm species (almost four times that of coconut) and suggested that it may also be the most efficient of all oilyielding plants. Native to West Africa, it grows in all tropical lowland regions of the world with adequate rainfall ( > 1.5 m per year) commonly from 15°N to 15°S latitude. 47 The species is produced commercially for its oil (see Table 3) and is reported to produce an average of 37 000 kg ha-~ of dry biomass annually. 4s The palm grows in deep, well-drained fertile soils that are free from iron concretions and tolerates a range of soil pt-I. 47 In the western hemisphere, the tree is well established in Brazil, 47 but the authors also are aware of plantations in other Latin American countries. It grows to a height of 9 m or more, and its crown may contain up to 40 or more fronds. As with Chinese tallow, the oil palm has both male and female flowers which are fertile at different times. Flowering starts at year two to five depending on strain, and fruiting is continuous thereafter. At full production, the palm bears up to 12 bunches or clusters of fruits per year. The bunch yield per palm is a minimum of 68 kg per plant (in Nigeria). This figure is for the Tenura variety of palm which is currently popular. The fruit has a thick mesocarp containing the palm oil, a thin shell (endocarp), and an intermediate size seed (compared with other varieties) or kernel (endosperm) which contains the seed oil. The palm oil consists mostly of palmitic (a saturated fatty acid) and oleic (an unsaturated fatty acid) acids, constituting on an average 41"2% and 42"5%, respectively, of the oil.47 The palm oil is used in margarine, cooking fats, bakery goods, ice cream, soaps, detergents, as a flux in tin plating, and in cold rolling of steel. The palm
59
Fuel and chemical co-production from tree crops TABLE 3
Oil Yields, World Production Figures, and Prices of Commercial Vegetable Oils Name
Yield a ( kg oil ha - l yr- i )
Tree crops Oil palm 2790 (2000-3900) d Chinese tallow 296 e ( 5 0 0 0 ) f Coconut 818 Conventional oil seed crops Sesame 420 Sunflower 392 Peanut 308 Linseed 193 Soybean 190 Corn (maize) -Rapeseed -Cottonseed --
1983 World production b ( 1000 metric tonnes of oil)
Average 1983 US price b.c ( 5 kg- ~ ofoil)
6 467
0"542
100 e
--
4 340
0'616
-5 810 3 300 2 168 13 010 445 h 5 200 3 340
-0-560g 0.756 0.635 0"514 0.496 1.219 0.553
Average factory yield.4t b From ref. 42. ' Prices of all vegetable oils rose dramatically during the last part of 1983. d Range of published values; 1 kg of palm oil = 1"084 liters. e Value is for mainland China where there is an industry; the reported yield is low since the crop is normally harvested after the rice harvest when much spoilage has occurred (H. W. Scheld, pers. commun.). f Estimated potential production of both tallow and oil.43 g From ref. 44. h Figure is US production only. The US produces about half the world's corn.
k e r n e l oil is similar to c o c o n u t oil 47 a n d c o n t a i n s a high p r o p o r t i o n of s a t u r a t e d fatty acids ( 5 0 % of the oil o n the a v e r a g e is lauric acid). It is u s e d in m a r g a r i n e , c o o k i n g oils, soaps, d e t e r g e n t s , c o s m e t i c s , a n d as an additive to lubricating oils o r as a d e t e r g e n t f o a m b o o s t e r . Oil p a l m is p r o p a g a t e d by s e e d a n d is g r o w n in a n u r s e r y for a b o u t a y e a r p r i o r to t r a n s p l a n t i n g to the field. T h e s t a n d a r d field spacing is an 8.7 m triangle w h i c h w o r k s o u t to be a b o u t 153 p a l m s ha-1.47 m lot of i n f o r m a t i o n is available a b o u t its n u t r i e n t r e q u i r e m e n t s . T h e tree grows on soils of r a t h e r low fertility, b u t fruiting m a y be r e d u c e d or t e r m i n a t e d by deficiencies of nitrogen, p o t a s s i u m , o r m a g n e s i u m . 47 In general, p a l m s are p r e c a r i o u s species; m a n y are e n d a n g e r e d (J. A. D u k e , pers. c o m m u n . ) . T h e oil p a l m is susceptible to a n u m b e r of fungal
60
M. Seibert, G. Williams, G. Folger, T. Milne
diseases and pests. Fungus is a problem especially during the seed germination and nursery stages, but fungicides are helpful. Most pests are controllable. Harvesting of the fruit is done by hand. The bunches must be inspected biweekly to prevent over-ripening of the fruit, which lowers the quality of the palm oil. Mechanized processes to obtain the two types of oils from the fruit are well established. 47 However, precautions must be taken to prevent an increase in free fatty acids due to bruising of the fruit, delay before processing, or the presence of contaminating water and other impurities in the processed oil. Palm kernel cake contains about 19% protein by weight and is used in livestock feed. Oil yields of 2000 kg ha-148 2790 kg ha-1,41 and 3922 kg ha-143 have been reported. ECONOMICS Appraisals of the economics of ethanol and oil production from tree crops are largely speculative at this time due to the unique and underdeveloped nature of the concepts. Preliminary economic evaluations of low-maintenance plantings of honeylocust on marginal land are provided by Williams 3 and Freedman, 49 but detailed analyses are lacking. The study by Freedman 49 estimated that honeylocust pods are worth up to S0.0946 dry kg-1, o r $267 ha-1 (at 2808 dry kg-1 ha-~, which is about half the yield necessary to produce the lowest amount of ethanol indicated in Table 2) as an ethanol feedstock, relative to corn at $3.00 per bushel and based on calculated yields of 271 liters of ethanol and 569 kg of stillage per dry metric ton of pods. While the above estimate of ethanol yield appears to be low, and thus non-competitive, relative to conventional sugar and starch row crops on agricultural lands, the aggregate production potential on marginal lands unsuited for annual row crops is considered highly significant. For example, Freedman estimates that 33.6 million hectares of land in the US classified as cropland in pasture are capable of producing, from honeylocust pods, about 25.4 billion liters of ethanol year-1 plus 53"5 million metric tons of 18% protein livestock feed. Freedman's preliminary economic evaluation is both representative of the state of knowledge and suggestive of the kinds of considerations which should be included in economic analyses of tree-crop systems for liquid fuel production. Empirical data are needed from carefully planned and monitored field trials of candidate cultivars. Accompanying data from research and development of technologies for harvesting and con-
Fuel and chemical co-production from tree crops
61
version are also required. As species characteristics, site requirements, cultural practices, extraction technologies, prospective yields, and allowable costs become better defined, the potential applicability of tree-crop production systems to the variable land resource base can be evaluated with improved precision. Tree-crop systems offer unique advantages that deserve consideration in economic analyses. For example, the tree-crops-to-alcohol concept envisions coproduction of perennial fruits together with woody biomass for fuel and/or other wood products, by-product animal feeds, and grazing or interspacial crop production. Multiple product benefits are expected to substantially enhance cost effectiveness. Through use of marginal lands, carefully integrated polycultural systems allow greater general land productivity while preventing the economic problems associated with land degradation and soil erosion. While low-intensity management is envisioned, production costs can vary in accordance with management intensity and labor costs. Likewise, scale of operations, which affect capital costs, can vary with the different objectives and capabilities of landowners. As in forest management, plantation establishment costs must be carried over the period until first harvest, but the period would typically be of much shorter duration and incremental returns can be obtained from the annual harvests. Inter- and intra-regional variations of such factors as climate, growing season, soils, and topography will influence species selection, management systems, yields, and the resultant costs and revenues. Regional differences of species and yields, market demand, establishment costs, and operating costs should be considered. Economics of scale for conversion facilities, which typically entail the heaviest load of front-end costs, may dictate resource supply requirements and allowable feedstock costs. The individual farmer, however, will be most concerned about net annual return per hectare. The production of vegetable oils from tree crops mirrors the uncertainties discussed above for alcohol production. In the case of Chinese tallow, one economic analysis46 predicted net yearly revenues of about $2700 ha- ~ ( 1979 dollars) to the farmer in a mature producing orchard based on sales of tallow, oil, feedmeal, and shells. This did not include plantation start-up costs, land rental, and carrying costs for 7 years which would total about $4000 ha- ~ (worst case assumed). In fact this is probably an optimistic estimate because the yield, upon which the analysis is based, is high compared to actual yields reported from Chinese tallow plantations currently operating in China (H. W. Scheld, pers. commun.; however, see footnote e of Table 3). Still, the species
62
Fuel and chemical co-production from tree crops
offers considerable potential since genetic optimization has not as yet been attempted. The situation for oil from African oil palm, however, is entirely different. There is a well established industry, and the average price in 1983 (Table 3) was S0.542 kg -1 at US port of entry. This converts to S0.50 liter- 1, which was almost three times the wholesale price of diesel fuel in the US at the time, but is quite comparable to that in Europe where fuel taxes are much higher. The price of vegetable oils, of course, is dictated by their use as an edible commodity. In the developing world the actual cost of producing palm oil, for example, is low compared to the US wholesale commodity price which is in fact controlled by the price of soybean oil (note production figures in Table 3). Thus, economics of using palm oil as a fuel in the tropical developing world versus export of the oil for cash to buy petroleum should be evaluated in light of the Brazilian experience of successfully converting excess sugar production to ethanol fuel production. We consider African oil palm very promising. AREAS OF UNCERTAINTY The purpose of this paper has been to point out a potential new source of renewable fuels and chemicals; namely, the production of fermentable sugars and other carbohydrates or the production of oils by the reproductive organs of certain tree species. Since we have emphasized the potential of such systems, it may be useful at this point to discuss the uncertainties. The primary uncertainty is the actual yields that might be obtained routinely under large area field conditions for different species under different geographical conditions. This can dramatically influence economics. With the possible exception of oil palm, yields for the new tree crops reported in this paper are questionable, thus necessitating experimental plantings. In this regard D. H. Scanlon (pers. commun, and ref. 31 ) of the Tennessee Valley Authority is conducting a 10-year study to accurately determine pod production yields of honeylocust. He has planted 848 grafted 'Millwood' and 'Calhoun' trees on six 0"5 to l'0 ha plots in six different states. One of us (G.W.) is participating in the study at the International Tree Crops Institute, USA, and has also put in a demonstration planting of persimmon in Kentucky. High yielding varieties need to be identified and propagated -- some progress has been made in this area. The effects of pests and diseases should be assessed. Another potential problem is that most species require 5 to 10 years to reach production, which can present cashflow problems for undercapitalized commercial ventures. Harvesting is also a
M. Seibert, G. Williams, G. Folger, T. Milne
63
potential labor- and cost-intensive requirement, but whole-tree-shakers and mechanical harvesters, currently used with some nut and fruit trees, could be adapted for use with energy-producing tree crops. Production of enough material to warrant building commercial fermentation/distillation facilities or oil-processing plants might also be a problem, but with sufficient planning this should not be insurmountable. Farm-size facilities should be investigated. CONCLUSIONS The uncertainties and the current oil glut notwithstanding, we are in a period of increasing environmental pressure to implement policies that can lead to a sustainable society. Failure to heed Nature's warning could result in collapse of civilization on a worldwide scale, as has been documented on local and regional scales many times in the p a s t y Tree-crop production for both food and fuel could be part of a worldwide strategy aimed at attaining sustainable and non-polluting supplies of resources. Finally, even if tree crops are inappropriate or uneconomic under US conditions, the conditions prevalent in the developing world may warrant application there, while systems for the developed world are being perfected. ACKNOWLEDGEMENTS The authors would like to thank J. A. Duke, P. Felker, D. H. Scanlon, H. W. Scheld, and R. J. Seibert for photographs of trees discussed in this paper and unpublished information. This work was supported by the Biomass Energy Technology Division, US Department of Energy. The genesis of this article, and the source of many of the citations given, was a workshop entitled Tree Crops f o r Energy Co-Production on Farms, held at Estes Park, Colorado, 12-14 November, 1980. The workshop was chaired by Thomas Milne and was guided by the SERI Tree Crop Task Force consisting of Michael Seibert, Robert Inman, Donald Hertzmark, and Carl Strojan. Limited copies of the proceedings are available from T. Milne at SERI. REFERENCES 1. Smith, J. R. (1950). Tree crops, a permanent agriculture, 2nd Edition, Devin-Adair Co., New York. First published by Harcourt Brace, New York (1929) and reprinted in paperback from Harper & Row, New York (1978).
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2. Brown, L. R. (1980). Food or fuel: New competition for the world's cropland, Worldwatch Paper 35, The Worldwatch Institute, Washington, DC. 3. Williams, G. (1981). Tree crops for energy production in Appalachia. In: Tree crops for energy production on farms, SERI/CP-622-1086, available from NTIS, Springfield, VA, pp. 7-20. 4. Westwood, M. N. (1978). Temperate-zone pomology, W. H. Freeman, San Francisco, pp. 228. 5. Ritter, C. M. & McKee~ G. W. (1964). The elderberry: History, classification, and culture, Pennsylvania State University Agricultural Experiment Station Bulletin 709, University Park, Pennsylvania, 22 pp. 6. Podems, M. & Bortz, B. (1975). Ornamentals for eating, Rodale Press, Emmaus, Pennsylvania, 67 pp. 7. Waino, W. W. & Forbes, E. (1941). The chemical composition of forest fruits and nuts from Pennsylvania. J. Agri. Res., 62,627-35. 8. Park, B. C. (1942). The yield and persistence of wildlife food plants. J. Wildlife Management, 6, 118-20. 9. Slate, G. L. (1955). Minor fruits. Natrl Hort. Magazine, 34 (3), 139-49. Also a letter from G. L. Slate, New York State Agric. Expt. Station to J. W. Hershey, Downingtown, PA, 16 July, 1951 (for a copy contact Gregory
Williams). 10. Detwiler, S. B. (1947). Notes on honeylocust, Washington, DC, USDA Soil Conservation Service, 197 pp. 11. Williams, G. (1980). Honeylocust, an update, North American Pomona, 13, 235-7. 12. Thomas, C. C. (1924). The Chinese jujube, USDA Bulletin No. 1215, US Government Printing Office, Washington, DC, 31 pp. 13. Felker, E, Clark, P. R., Osborn, J. E & Cannell, G. H. (1981). Utilization of mesquite (Prosopis spp.) pods for ethanol production. In: Tree crops for energy co-production on farms, SERI/CP-622-1086. Available from NTIS, Springfield, VA, pp. 65-78. 14. Chaffield, C. & McLaughlin, L. I. (1928). Proximate composition of fresh fruits, USDA Circular No. 50, Washington, DC, US Government Printing Office, 19 pp. 15. Barnhart, E. (1980). Personal communication. New Alchemy Institute, East Falmouth, Massachusetts. 16. Bargandzhiya, A. G. (1977). Biological characteristics of flowering and fruiting of oriental persimmon, Byulleten' vsesoyuznogo ordena Lenina Instituta Rastenievodstva imeni N.I. Vavilova, No. 68, 36-38. In Russian. 17. Claypool, J. V. (1980). Personal communication, St. Elmo, Illinois. 18. Jaynes, R. A. (ed.) (1979). Nut tree culture in North America, Northern Nut Growers Association, Hamden, Connecticut, 466 pp. 19. Beaverlodge Nursery (1979). Hardy plants for Northern gardens, Beaverlodge, Alberta, 45 pp. 20. Smock, R. N. & Neubert, A. M. (1950). Apples and apple products, Interscience Publishers, New York.
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21. Jacobs, P. B. & Newton, H. E (1938). Motor fuels from farm products, USDA Miscellaneous Publication No. 327, US Government Printing Office, Washington, DC. 22. Shallenberger, R. S. (1974). Occurrence of various sugars in foods. In: Sugars in nutrition, H. L. Sipple and K. W. McNutt (eds), Academic Press, New York, pp. 67-80. 23. Woodroof, J. G. (1979). Tree nuts: production, processing, products, 2nd edition, AVI Publishing, Westport, Connecticut, 731 pp. 24. Payne, J. A. (1978). Chinese chestnut production in the Southeastern United States: Practice, problems, and possible solutions. In: Proceedings of the American chestnut symposium, West Virginia University Books, Morgantown, West Virginia, pp. 20-4. 25. Agricultural Research Service Northeastern Region (1976). Growing black walnuts for home use, U S D A Leaflet No. 525, US Government Printing Office, Washington, DC, 9 pp. 26. Halls, L. K. (ed.) (1977). Southern fruit-producing woody plants used by wildlife, USDA Forest Service General Technical Report SO-16, US Government Printing Office, Washington, DC, 235 pp. 27. Langworthy, C. E & Holmes, A. D. (1917). The American papaw and its food value. J. Home Econ., 9, 505-11. 28. Scanlon, D. H. (1981). A case study of honeylocust in the Tennessee Valley region. In: Tree crops for energy co-production on farms, SERI/CP-6221086, available from NTIS, Springfield, VA, pp. 21-31. 29. Williams, G. & Merwin, M. L. (1983). Energy- and soil-conserving perennial crops for marginal land in temperate climate. In: Environmentally sound agriculture, W. Lockeretz (ed.), Praeger Publishers, New York, pp. 317-32. 30. US Forest Service (1965). Silvics of forest trees of the United States, Agriculture Handbook No. 271, USDA Forest Service, pp. 187-201. 31. Tennessee Valley Authority (1984). TVA biomass fuels update II, TVA OACD-84/3, available from NTIS, Springfield, VA, pp. 13-14. 32. McDaniel, J. C. ( 1981 ). A plant breeder looks at some American tree crops: Morus, Gleditsia, and Diospyros. In: Tree crops for energy co-production on farms, SERI/CP-622-1086, available from NTIS, Springfield, VA, pp. 113-18. 33. Smith, J. L. & Perino, J. V. ( 1981 ). Osage orange (Maclura pomifera) history and economic uses. Economic Botany, 35 ( 1 ), 24-4. 34. Clopton, J. R. & Roberts, A. (1949). Osage orange -- A potential source of edible oil and other industrial raw materials. J. Oil Chem. Soc., 26,470-2. 35. Hokes, J., Chism, G. & Hansen, E M. T. (1976). Osage orange -- A source of proteolytic enzyme. Ohio Report on Research and Development, 61 ( 1 ), 11-13. 36. Leakey, C. L. A. (1977). Breadfruit reconaissance study in the Caribbean region, Monograph of CIAT/Inter-American Development Bank. 37. Avgerinos, G. C. & Wang, D. I. C. (1981). Utilization of mesquite and
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38. 39.
40.
41. 42. 43.
44. 45.
46.
47. 48. 49.
50.
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honeylocust pods as feedstocks for energy production. In: Tree crops for energy co-production on farms, SERI/CP-622-1086, available from NTIS, Springfield, VA, pp. 209-17. Ogden, R. L. (1983). Attempt to ferment sugars in locust tree bean. Agroforestry Review, 3 (3 and 4), 5. Kaufman, K. R. (1984). Testing of vegetable oils in diesel engines. In: Fuels and chemicals from oilseeds. Technology and policy options, E. B. Schultz, Jr. and R. P. Morgan (eds), AAAS Symposium 91, Westview Press, Boulder, CO, pp. 143-75. Weisz, P. B., Haag, W. O. & Rodewald, G. (1979). Catalytic production of high-grade fuel (gasoline) from biomass compounds by shape-selective catalysis. Science, 206, 57-8. Hodge, W. H. (1975). Oil-producing palms of the w o r l d - A review. Principes, 19 (4), 119-36. Emergy, W. L. (ed.) (1984). 1984 Commodity Year Book, Commodity Research Bureau, Jersey City, NJ. Scheld, H. W., Cowles, J. R., Engler, C. R., Kleiman, R. & Schultz, Jr., E. G. (1984). Seeds of the Chinese tallow tree as a source of chemicals and fuels. In: Fuels and chemicals from oilseeds. Technology and policy options, E. B. Schultz, Jr., and R. P. Morgan (eds), AAAS Symposium 91. Westview Press, Boulder, CO, pp. 81-107. FAO (1984). 1983 FAO Production Yearbook, Vol. 37, FAO of the UN, Rome, pp. 307. Cowles, J. R. & Scheld, H. W. (1984). Cultural and management practices for the Chinese tallow tree as a biomass fuel source, Final DOE Report, Contract No. ET-78-G-01-3037. Scheld, H. W., Bell, N. B., Cameron, G.N., Cowles, J. R., Engler, C. R., Krikorian, A. D. & Schultz, Jr, E. B. (1981). The Chinese tallow tree as a cash and petroleum-substitute crop. In: Tree crops for energy coproduction on farms, SERI/CP-622-1086, available from NTIS, Springfield, VA, pp. 97-111. Opeke, L. W. (1982). Tree crops, John Wiley and Sons, New York, pp. 251-74. Duke, J. A. (1977). Palms as energy sources: A solicitation. Principes, 21, 60-2. Freedman, D. (1981). Preliminary analysis of the potential for ethanol production from honeylocust pods. In: Tree crops for energy co-production on farms, SERI/CP-622-1086, available from NTIS, Springfield, VA, pp. 121-34. Brown, L. R. (1982). R-and-D for a sustainable society. Am. Scientist, 70 (1), 14-17.
Fuel and Chemical Co-Production From Tree Crops Michael Seibert*, Gregory Williamst, Gray Folger* and Thomas Milne* *Solar Energy Research Institute,$ 1617 Cole Boulevard, Golden, CO 80401, USA ?International Tree Crops Institute, USA, Inc., Route 1, Gravel Switch, KY 40328, USA (Received: 13 July, 1985)
A BSTRACT A concept for the sustainable production of fuels and chemicals from fermentable and oleaginous substrates produced on an annual basis by the reproductive organs (pods, fruits, nuts, berries, etc.) of'tree crops' is presented. The advantages of tree-crop systems include suitability for use on marginal land, potential productivity equivalent to row crops, minimal maintenance and energy input requirements, environmental compatibility, and the possibility of coproduct production. Disadvantages are possible high establishment costs, a long growth period until production commences, and difficulties in harvesting or storage; however, these provide opportunities for potential research impact. Honeylocust, mesquite, persimmon, osage orange, and Chinese tallow are examined as potential US tree-crop species. Other species, including breadfruit and African oil palm, are suggested as tree-crop candidates for the tropical developing world. Fermentation or extraction of treecrop organs and the economics of tree-crop systems are also discussed. Currently, the greatest area of uncertainty lies in the actual pod or fruit yields that can be expected from large tree farms under real life conditions. However, ballpark ethanol yield estimates of 650 to 34701itresha -I (69-371gallonsacre -l) and oil yields up to 3900 kg ha- l (4227 litres ha- i) justify further consideration of treecrop systems. Key words: Tree crops, alcohol, oil, biomass, marginal land, fuels and chemicals. :~Operated by the Midwest Research Institute for the US Department of Energy under contract No. DE-AC02-83CH 10093. 49 Biomass 0144-4565/86/S03.50 - © Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain
50
M. Seibert, G. Williams, G. Folger, T. Milne
INTRODUCTION About 50 years ago Smith 1 introduced a new agricultural concept which he termed 'tree crops'. The notion was to develop a sustainable foodproduction system based upon the use of previously uncommercialized trees grown for an annual crop of pods, fruits, nuts, or berries. The use of tree crops for fuel and chemical production, an extrapolation of Smith's ideas, is examined here. It should be emphasized at this point that we use the term 'tree crops' in the historical sense, in contrast to 'crops of trees' utilized in silviculture energy plantations where wood or energy-rich components found in the wood are the primary products. This paper considers trees that produce large amounts of sugars, other carbohydrates, or oleaginous materials in specialized organs on a perennial basis. These materials can be viewed as a source of fermentable substrates for ethanol production, oils for diesel fuel substitutes, or raw materials for the chemical industry
A D V A N T A G E S OF TREE CROPS The recent resurgence of interest in fuel alcohol production from fermentation of traditional agricultural crops, particularly grains, has heightened concern for the ecological and policy implications of a perceived 'food vs fuel' conflict 2 and for the conversion of additional land to grain production. Alcohol obtained from tree-crop sources circumvents this potential conflict because the trees can be grown on marginal land, including that with poor soil, steep terrain, and semi-arid conditions unsuitable for row crops. Oil-producing tree crops exhibit similar advantages. As will be documented later, the potential annual productivity of certain species is equivalent to or higher than row crops. In addition, tree cultivation may require minimal maintenance and energy input; is conducive to soil amelioration, erosion prevention, water retention, and wind protection; provides an environment suitable for wildlife and other multiple uses; and can produce many coproducts such as lumber, firewood, protein, specialized chemicals, honey, and fiber, to name a few. SPECTRUM OF P O T E N T I A L C A R B O H Y D R A T E - P R O D U C I N G WOODY SPECIES Table 1 presents a list of potential, woody perennials which have been considered as candidate tree-crop species in the US. 3, All grow over
Fuel and chemical co-production from tree crops
51
large portions of the country (though Eastern species are emphasized), and all produce large numbers of reproductive organs that contain large quantities of sugars and/or other carbohydrates. For convenience the table is divided qualitatively into three feasibility categories based on potential equivalent alcohol yields ha-land tree maintenance requirements necessary for high yields. Most of the candidates can be eliminated based on low potential alcohol production yields and high maintenance requirements. More information on the 'high feasibility' species of Table 1 is given in Table 2. Ethanol yields for corn are noted for comparison. The estimated ethanol yields in Table 2 (except mesquite) are based on literature values for reproductive organ yields and composition (% sugars and other carbohydrates), theoretical conversion of 1 mole of
TABLE 1 Feasibility Ranking of Selected Woody Perennials for Ethanol Production a
High feasibility ( high yields, b low maintenance")
Minimal feasibility (high yields, high maintenance)
Blackberry (4) Blueberry (4) Elderberry (5, 6) Hawthorn (7, 8) Highbush cranberry (6,7,9) Honeylocust (10, 11) Jujube (12) Mesquite (13) Mulberry (14, 15) Oriental persimmon (14,16) Persimmon (1, 14, 17) Raspberry (4, 14) Sawtooth oak (7, 18) Serviceberry (6, 7, 19)
Apple (4, 20) Cranberry (4) Grape (4, 9) Peach (4, 14) Pear (4, 21) Plum (4, 14) Sour cherry 4, 22) Sweet cherry (4, 22)
Unfeasible (low yields) American beech (8, 23) Chinese chestnut (18, 23, 24) Currant (4, 6, 14) Dogwood (7, 8) Eastern black walnut (23,25) Gooseberry (4, 6, 14) Hazel (7, 18) Holly (8, 26) Various oak species (7, 18) Papaw (27) Pecan (18, 23) Persian walnut ( 18, 23) Shagbark hickory (23, 26) Wild plum (8)
a Numbers in parentheses indicate references. b High yields are defined as estimated potential ethanol yields of >/468 liters ha-~ (50 gallons acre-1) though maximum values in almost all cases were over 935 liters ha(100 gallons acre-1). Low yields are defined as a maximum potential of ~<468 liters ha-1. c Maintenance was judged on requirements for spraying (insect and disease control) and pruning to maintain high yields.
Small tree/pome
Tree/acorns
281 - 1216
c. 655 (at 10 yr old)
Precocious
Arid land species Subject to animal damage; fruit stores poorly Difficult to establish; subject to late frost damage; hardiness problems; fruit stores poorly Drought resistant and tolerates poor site conditions; difficult to establish but are hardier and flower later than D. kaki; subject to wilt disease in the south; fruit stores poorly Precocious, annual bearing
Drought resistant and tolerates poor site conditions; tends to alternate bear; susceptible to mimosa webworm
Precocious
Requires acid soil; subject to animal damage; fruit stores poorly Precocious; fruit stores poorly
Subject to animal damage; fruit stores poorly
Comments
"See Table 1 for references. b For comparison, corn at 100 bu acre- t would yield 2067 liters ha- ~of 100% ethanol based on an 85% fermentation yield. To convert liters ha- Lto gallons acre t, divide by 9.353.
Sawtooth oaks (Quercus acutissima Carr.) Serviceberry (A melanchier sp. Med.)
1309-3367
1496-3741
Tree/pod
Tree/berry
c. 1029 561-2619
Small tree/pome Small tree/drupe
c. 1403 882-1510 c. 935 748-5986
374-1029
Small tree/drupe
Tree/drupe Tree/pod Tree/drupe aggregate Tree/berry
286-1496
Shrub/berry
Jujube (Ziziphusjujuba Mill.) Mesquite (Prosopissp.) Mulberry (Morus sp. L.) Oriental persimmon (Diospyros kaki L.) Persimmon (Diospyros virginiana L.)
1964 maximum
Vine or shrub/drupe
Blackberry, raspberry (Rubus sp. L) Blueberry ( Vaccinium corymbosum L.) Elderberry (Sambucus canaensis L.) Hawthorn ( Crataegus sp. L) Highbush cranberry ( Viburnum trilobum Marsh.) Honeylocust ( Gleditsia triancanthos L.)
Estimated ethanol h yield ( litres ha - ~yr t )
Plant/organ type
Name
TABLE 2 E s t i m a t e d E t h a n o l Yields from, Selected W o o d y Perennials"
e~
L~ t'~
M. Seibert, G. Williams, G. Folger, T. Milne
53
sugar (glucose or fructose) to 2 moles of ethanol, and fermentation yields of 85%. These specific values used to convert sucrose, starch, and invert sugar to ethanol were 0.579, 0.609, and 0.551 litres kg -1, respectively. For mesquite, osage orange, and breadfruit we used the same assumptions except that 0.462 litres kg-1 was used to convert carbohydrate to ethanol based on the assumption that carbohydrate in these species is similar to that in corn. We should emphasize that the ethanol yield values given should be taken as best current estimates. In many cases the yields are based on production results from small numbers of trees for which the growth conditions were uncontrolled or undocumented. With this important caveat we have chosen honeylocust, mesquite, and persimmon from Table 2 as three species warranting further investigation because they tolerate relatively poor site conditions.
CARBOHYDRATE SPECIES WARRANTING ADDITIONAL INVESTIGATION In this section we shall give relevant background information on each of the species identified in the last section as warranting further investigation.
Honeylocust (Gleditsia triacanthos L.) Honeylocust is native to the central United States and can be found east of the Mississippi from Alabama and Georgia to New England. Superior thornless varieties ('Millwood' and 'Calhoun') are subject to few pests and diseases (at least in the wild). The mimosa webworm, however, can inflict serious damage. Deer have killed many thornless trees planted for yield trials, but hanging bars of deodorant soap in the trees seems to keep them away. Honeylocust can tolerate poor growing conditions with yearly precipitation levels as low as 0.5 m. The trees have an open foliage canopy, which is advantageous for grass or (nitrogen-fixing) legume production underneath. They are either predominantly pistillate (female, pod producing) or staminate (male, pollen producing) and can be propagated vegetatively. For good pod production some staminate trees should be available. Planting densities of 86 to 111 trees ha- ~ are probably optimal, and fertilization is required especially if the land is also to be used for grazing of livestock. Significant production of long flat, dark brown pods starts when vegetatively propagated trees are about 6 years old. Trees propagated in this manner bear at a younger age than seedlings. Nine-year-old 'Millwood' trees (99ha -~) have reportedly
54
M. Seibert, G. Williams, G. Folger, T. Milne
produced up to 8068 dry kg of pods ha-l, though they tend to alternately bearY Pruning may minimize this problem. Pods could be harvested mechanically using tree-shaking machines or chemical pod abscission agents and the pods raked up with conventional haying equipmentfl9 Total pod sugar contents of up to 42.3% and protein contents up to 15.4% have been recorded (dry weight). Cattle feeding trials have demonstrated that the pods can be substituted for oats on a weight basis. The tree itself is not a particularly fast grower but can attain a height of 21 to 24 m at maturity (120 years). Honeylocust has been used extensively in Midwestern shelter belts. Further information can be found in reports by Scanlon, 28 Williams and Merwin, 29 and the USDA Forest Service.3° Large-scale planting trials are underway. 31
Mesquite (Prosopis sp.) Mesquite is a nitrogen-fixing, cold-sensitive, salt-tolerant shrub or tree found in marginal, semi-arid land in the southwestern United States. The wood commands premium prices as both lumber and firewood. Currently it is also being converted to charcoal. However, prior to 1978 the only US government-related effort with mesquite was an eradication program to eliminate it from range land. The tree shows some susceptibility to psyllid insect and mistletoe damage. Based on 6% moisture content, pod yields of 4000 kg ha-~ year-~ should be possible under range land conditions; and at 200 trees ha- 1,where water is not limiting, 10 000 kg of pods ha-~ year-1 might be expected. Harvesting could be done as with honeylocust. The pods contain from 13% to 36% sucrose and from 11% to 17% protein (at 2% water content). High yielding, thornless trees should be selected and propagated. In this respect, some progress has been made initiating shoots by plant tissue culture techniques (E Felker, pers. commun.). More detailed information is available from Felker et al. ~L~
Persimmon (Diospyros virginiana) The common persimmon is adaptable to poor soil conditions but requires more precipitation than honeylocust. It is found in the southeastern United States from southern Florida to southern Connecticut and west to Texas and Kansas. It can be propagated vegetatively by grafting on to established rootstock. Trees should be planted at a density of about 200 trees ha- 1. A few male trees should be included for pollen, but cultivars that set nearly full-sized seedless fruit parthenocarpically are known. This characteristic may offer a distinct advantage
Fuel and chemical co-production from tree crops
55
for processing. Trees reach a height of 15 to 32 m (shorter under dry conditions) and are important commercially for their wood. Several insect species can attack persimmon but do not cause much damage. However, the persimmon borer (Sannina uroceriformia)could be a very serious pest in large-scale plantings, and a fungus (Cephalosporium diospyri) disease has been known to kill many trees in the southern states. Deer browsing has been suggested as a potential problem. In reality it has not been a concern during yield trials in Kentucky; however, opossums and other wildlife may be a problem. Some tree selection has been accomplished over the past century, but emphasis has been on fruit quality for human consumption. Some 10-year-old trees will produce fruit, but maximum production occurs at age 25 to 50 years. Grafted trees begin fruiting after 3 to 4 years. The tree does not tend to alternately bear to a great extent.The fruit ripens over a relatively long period (about a m o n t h ) a n d does not store well. However, some cultivars produce fruit that can be dried without molding. More information can be found in References 29, 30, 32.
OTHER CANDIDATE CARBOHYDRATE-PRODUCING SPECIES NOT PREVIOUSLY CONSIDERED Osage orange (Maclura pomifera) is native to Arkansas, Texas, and Oklahoma but grows as far north as New England. It produces an unpalatable, multiple fruit that is a globose, yellowish-green structure weighing about 0.45 kg, 80% of which is water. 33 The tree starts to bear at 10 years of age, lives 150 years or more, and can reach a height of 9-12 m. It can be propagated vegetatively and is one of the most insectand disease-resistant tree species in North America. The fruits fall to the ground after the first hard frost and store fairly well for several months at cool temperatures. One of us (T.M.) has observed a yield of 300 fruits produced per female tree in Missouri, which is less than reported values of up to 450 kg of fruit per tree? 4 The fruit contains about 15% sugars and 7% other carbohydrates on a dry weight basis. The tree can be planted at very high density, but if planting densities of 100 trees ha(10 m centers) are used, 1073 liters ha -~ year -~ of ethanol could be obtained using previous assumptions. Though the estimated yield is rather low, there has been no selection of osage orange for fermentable fruit carbohydrates, and other coproducts representing 78% of the fruit dry weight are potentially valuable. For example, the fruit contains proteolytic enzymes that have potentially important economic uses and many secondary metabolites, oils (about 18.5% on a dry weight basis),
56
M. Seibert, G. Williams, G. Folger, T, Milne
and resins 35 which may also be of use. The wood is very hard, resists rotting, and is still used for fence posts. Probably the species' most important use in the past has been as 'living fences'. Tropical species provide a wealth of genetic material and should not be overlooked. Possible US applications may be suitable for Puerto Rico or Hawaii, but certainly there is much potential in the developing world where fuel requirements are great, oil imports expensive, and labor is abundant and inexpensive. The positive Brazilian experience using sugar cane as the raw material for fermentation to ethanol is well known, but breadfruit (Artocarpus communis J. R. and G. Forst) may provide another opportunity. 36 The tree grows well in tropical lowlands that receive adequate rainfall. In the Caribbean the only serious disease problem is caused by Rosellinia sp., which is associated with poor agricultural practices. Insect problems seem to be minor. Fungus is a much more serious problem in the South Pacific. The fruit weighs about 0.9 kg (wet weight) on average and contains from 68% to 80% water. On a dry basis, it contains between 72% and 86% carbohydrate. Production yields of 6993 to 30 033 wet kg ha- 1year- 1 depending on water availability have been reported. This would give 655-2850 liters of ethanol ha-l year ~ using the previous criteria. The tree is already grown in the Caribbean as a source of food (though not on plantations as might be necessary for use as a fuel crop). Finally, the species can be propagated from root suckers.
FERMENTATION Avgerinos and Wang 37 have fermented both honeylocust and mesquite pods using a mixed culture of two thermophilic bacteria. In the case of mesquite the organism utilized up to 83% of the total fermentable carbohydrates (which represents 1.5 to 1.8 times the sugar contents mentioned earlier in this paper) with the production of ethanol at 80% of the theoretical maximum yield. Preliminary fermentation results with honeylocust were disappointing due perhaps to a substance in the pods that inhibited these particular bacteria. Ogden 3~ has also reported limited success in attempting to ferment honeylocust pods.
OIL-PRODUCING TREE CROPS Oil seed crops have considerable potential as a renewable source of petroleum substitutes. Tests indicate that vegetable oils may be used
Fuel and chemical co-production from tree crops
57
directly as a diesel fuel substitute in indirect injection but not in direct injection diesel engines (the most c o m m o n type). 39 However, these oils can be modified to substitute for petroleum-based products. For example, they can be trans-esterified to function in direct injection diesel engines, or they can be converted to gasoline by exposure to a zeolite catalyst at high temperature and moderate pressure. 4° In this section we will discuss two promising tree species that could have significant impact on the fuel and chemical industries. Both are currently being produced commercially outside of the US, but not for fuel use. For comparison, Table 3 shows yields, world production figures, and prices of various vegetable oils. Chinese tallow
The Chinese tallow tree, Sapium sebiferum (L.) Roxb., a Euphorb, is a native of China, where it has been cultivated for as long as a thousand years. In the US, it grows on marginal lands along most of the coastal region from North Carolina to south Texas and in southern California. It is a deciduous, subtropical species that is small (10-14 m) and shortlived (40 to 50 years). The tree grows in low, wet areas, thrives in poorly drained soil, and is salt tolerant. In addition, its deep root system gives it considerable drought tolerance. Insects and disease have not been problems perhaps due to the high levels of phenolics in the leaves and sap. However, defoliation due to leaf cutting ants (Atta texana) and the yellow-striped army worm (Spodoptera ornithogallis)has been observed in south Texas. Neither is seen as a major threat, a5 Fruits develop in clusters of three- or four-celled capsules which contain a single pea-sized white seed. A white greasy mass, or vegetable tallow, covers the seed, and the seed kernel contains a liquid oil called stillingia oil. Both are triglycerides, the former containing mostly saturated fatty acids and the latter, unsaturated fatty acids. The tallow is very similar in properties to cocoa butter. It is edible and has been used in candle and soap making. The seed oil is similar to drying oils. It has been used as an illuminating or lubricating oil, in paints and finishes, and in cosmetics. Both the tallow and the oil are currently used in the Chinese chemical industry (H. W. Scheld, pers. commun.). The composition of the seed is highly variable with tallow content ranging from 11% to 32% and the oil content from 15% to 30% by weight. The seed also contains a thiamine-rich but lysineand methionine-deficient protein ( - 1 1 % ) suitable as a livestock feed supplement or for use in baking flour. Seed yields of 11 205 kg ha- t at 395 trees ha- ~ have been reported and estimates of up to 5043 kg of tallow plus oil h a - 1 have been suggested. 43,46
58
M. Seibert, G. Williams, G. Folger, T. Milne
Chinese tallow is also attractive in the number of products that the tree provides. Besides the seed products discussed above, the tree can be used as a fuelwood crop since it is adaptable to close spacing and direct seeding, it grows rapidly for the first 6 years (10-15 dry tonnes ha -~ year -1 in densely planted stands; dry wood from 2-year-old trees averages 4225 kcal kg-~), and coppices. 45 Older trees partition most of their photosynthate into seed production, though seedset can commence as early as the third season of growth. On the other hand, the tree is spread rapidly by birds ingesting and disseminating seeds and thus could become a 'weed' problem. The leaves produce a black dye used by the Chinese for silk and could be a commercial source of industrial tannins. About 112 kg of honey, with a wholesale value of S123 has also been reported per hectare. 43
African oil palm Hodge 4t has reported that the African oil palm (Elaeis guineensis Jacq.) is the most efficient oil-producing palm species (almost four times that of coconut) and suggested that it may also be the most efficient of all oilyielding plants. Native to West Africa, it grows in all tropical lowland regions of the world with adequate rainfall ( > 1.5 m per year) commonly from 15°N to 15°S latitude. 47 The species is produced commercially for its oil (see Table 3) and is reported to produce an average of 37 000 kg ha-~ of dry biomass annually. 4s The palm grows in deep, well-drained fertile soils that are free from iron concretions and tolerates a range of soil pt-I. 47 In the western hemisphere, the tree is well established in Brazil, 47 but the authors also are aware of plantations in other Latin American countries. It grows to a height of 9 m or more, and its crown may contain up to 40 or more fronds. As with Chinese tallow, the oil palm has both male and female flowers which are fertile at different times. Flowering starts at year two to five depending on strain, and fruiting is continuous thereafter. At full production, the palm bears up to 12 bunches or clusters of fruits per year. The bunch yield per palm is a minimum of 68 kg per plant (in Nigeria). This figure is for the Tenura variety of palm which is currently popular. The fruit has a thick mesocarp containing the palm oil, a thin shell (endocarp), and an intermediate size seed (compared with other varieties) or kernel (endosperm) which contains the seed oil. The palm oil consists mostly of palmitic (a saturated fatty acid) and oleic (an unsaturated fatty acid) acids, constituting on an average 41"2% and 42"5%, respectively, of the oil.47 The palm oil is used in margarine, cooking fats, bakery goods, ice cream, soaps, detergents, as a flux in tin plating, and in cold rolling of steel. The palm
59
Fuel and chemical co-production from tree crops TABLE 3
Oil Yields, World Production Figures, and Prices of Commercial Vegetable Oils Name
Yield a ( kg oil ha - l yr- i )
Tree crops Oil palm 2790 (2000-3900) d Chinese tallow 296 e ( 5 0 0 0 ) f Coconut 818 Conventional oil seed crops Sesame 420 Sunflower 392 Peanut 308 Linseed 193 Soybean 190 Corn (maize) -Rapeseed -Cottonseed --
1983 World production b ( 1000 metric tonnes of oil)
Average 1983 US price b.c ( 5 kg- ~ ofoil)
6 467
0"542
100 e
--
4 340
0'616
-5 810 3 300 2 168 13 010 445 h 5 200 3 340
-0-560g 0.756 0.635 0"514 0.496 1.219 0.553
Average factory yield.4t b From ref. 42. ' Prices of all vegetable oils rose dramatically during the last part of 1983. d Range of published values; 1 kg of palm oil = 1"084 liters. e Value is for mainland China where there is an industry; the reported yield is low since the crop is normally harvested after the rice harvest when much spoilage has occurred (H. W. Scheld, pers. commun.). f Estimated potential production of both tallow and oil.43 g From ref. 44. h Figure is US production only. The US produces about half the world's corn.
k e r n e l oil is similar to c o c o n u t oil 47 a n d c o n t a i n s a high p r o p o r t i o n of s a t u r a t e d fatty acids ( 5 0 % of the oil o n the a v e r a g e is lauric acid). It is u s e d in m a r g a r i n e , c o o k i n g oils, soaps, d e t e r g e n t s , c o s m e t i c s , a n d as an additive to lubricating oils o r as a d e t e r g e n t f o a m b o o s t e r . Oil p a l m is p r o p a g a t e d by s e e d a n d is g r o w n in a n u r s e r y for a b o u t a y e a r p r i o r to t r a n s p l a n t i n g to the field. T h e s t a n d a r d field spacing is an 8.7 m triangle w h i c h w o r k s o u t to be a b o u t 153 p a l m s ha-1.47 m lot of i n f o r m a t i o n is available a b o u t its n u t r i e n t r e q u i r e m e n t s . T h e tree grows on soils of r a t h e r low fertility, b u t fruiting m a y be r e d u c e d or t e r m i n a t e d by deficiencies of nitrogen, p o t a s s i u m , o r m a g n e s i u m . 47 In general, p a l m s are p r e c a r i o u s species; m a n y are e n d a n g e r e d (J. A. D u k e , pers. c o m m u n . ) . T h e oil p a l m is susceptible to a n u m b e r of fungal
60
M. Seibert, G. Williams, G. Folger, T. Milne
diseases and pests. Fungus is a problem especially during the seed germination and nursery stages, but fungicides are helpful. Most pests are controllable. Harvesting of the fruit is done by hand. The bunches must be inspected biweekly to prevent over-ripening of the fruit, which lowers the quality of the palm oil. Mechanized processes to obtain the two types of oils from the fruit are well established. 47 However, precautions must be taken to prevent an increase in free fatty acids due to bruising of the fruit, delay before processing, or the presence of contaminating water and other impurities in the processed oil. Palm kernel cake contains about 19% protein by weight and is used in livestock feed. Oil yields of 2000 kg ha-148 2790 kg ha-1,41 and 3922 kg ha-143 have been reported. ECONOMICS Appraisals of the economics of ethanol and oil production from tree crops are largely speculative at this time due to the unique and underdeveloped nature of the concepts. Preliminary economic evaluations of low-maintenance plantings of honeylocust on marginal land are provided by Williams 3 and Freedman, 49 but detailed analyses are lacking. The study by Freedman 49 estimated that honeylocust pods are worth up to S0.0946 dry kg-1, o r $267 ha-1 (at 2808 dry kg-1 ha-~, which is about half the yield necessary to produce the lowest amount of ethanol indicated in Table 2) as an ethanol feedstock, relative to corn at $3.00 per bushel and based on calculated yields of 271 liters of ethanol and 569 kg of stillage per dry metric ton of pods. While the above estimate of ethanol yield appears to be low, and thus non-competitive, relative to conventional sugar and starch row crops on agricultural lands, the aggregate production potential on marginal lands unsuited for annual row crops is considered highly significant. For example, Freedman estimates that 33.6 million hectares of land in the US classified as cropland in pasture are capable of producing, from honeylocust pods, about 25.4 billion liters of ethanol year-1 plus 53"5 million metric tons of 18% protein livestock feed. Freedman's preliminary economic evaluation is both representative of the state of knowledge and suggestive of the kinds of considerations which should be included in economic analyses of tree-crop systems for liquid fuel production. Empirical data are needed from carefully planned and monitored field trials of candidate cultivars. Accompanying data from research and development of technologies for harvesting and con-
Fuel and chemical co-production from tree crops
61
version are also required. As species characteristics, site requirements, cultural practices, extraction technologies, prospective yields, and allowable costs become better defined, the potential applicability of tree-crop production systems to the variable land resource base can be evaluated with improved precision. Tree-crop systems offer unique advantages that deserve consideration in economic analyses. For example, the tree-crops-to-alcohol concept envisions coproduction of perennial fruits together with woody biomass for fuel and/or other wood products, by-product animal feeds, and grazing or interspacial crop production. Multiple product benefits are expected to substantially enhance cost effectiveness. Through use of marginal lands, carefully integrated polycultural systems allow greater general land productivity while preventing the economic problems associated with land degradation and soil erosion. While low-intensity management is envisioned, production costs can vary in accordance with management intensity and labor costs. Likewise, scale of operations, which affect capital costs, can vary with the different objectives and capabilities of landowners. As in forest management, plantation establishment costs must be carried over the period until first harvest, but the period would typically be of much shorter duration and incremental returns can be obtained from the annual harvests. Inter- and intra-regional variations of such factors as climate, growing season, soils, and topography will influence species selection, management systems, yields, and the resultant costs and revenues. Regional differences of species and yields, market demand, establishment costs, and operating costs should be considered. Economics of scale for conversion facilities, which typically entail the heaviest load of front-end costs, may dictate resource supply requirements and allowable feedstock costs. The individual farmer, however, will be most concerned about net annual return per hectare. The production of vegetable oils from tree crops mirrors the uncertainties discussed above for alcohol production. In the case of Chinese tallow, one economic analysis46 predicted net yearly revenues of about $2700 ha- ~ ( 1979 dollars) to the farmer in a mature producing orchard based on sales of tallow, oil, feedmeal, and shells. This did not include plantation start-up costs, land rental, and carrying costs for 7 years which would total about $4000 ha- ~ (worst case assumed). In fact this is probably an optimistic estimate because the yield, upon which the analysis is based, is high compared to actual yields reported from Chinese tallow plantations currently operating in China (H. W. Scheld, pers. commun.; however, see footnote e of Table 3). Still, the species
62
Fuel and chemical co-production from tree crops
offers considerable potential since genetic optimization has not as yet been attempted. The situation for oil from African oil palm, however, is entirely different. There is a well established industry, and the average price in 1983 (Table 3) was S0.542 kg -1 at US port of entry. This converts to S0.50 liter- 1, which was almost three times the wholesale price of diesel fuel in the US at the time, but is quite comparable to that in Europe where fuel taxes are much higher. The price of vegetable oils, of course, is dictated by their use as an edible commodity. In the developing world the actual cost of producing palm oil, for example, is low compared to the US wholesale commodity price which is in fact controlled by the price of soybean oil (note production figures in Table 3). Thus, economics of using palm oil as a fuel in the tropical developing world versus export of the oil for cash to buy petroleum should be evaluated in light of the Brazilian experience of successfully converting excess sugar production to ethanol fuel production. We consider African oil palm very promising. AREAS OF UNCERTAINTY The purpose of this paper has been to point out a potential new source of renewable fuels and chemicals; namely, the production of fermentable sugars and other carbohydrates or the production of oils by the reproductive organs of certain tree species. Since we have emphasized the potential of such systems, it may be useful at this point to discuss the uncertainties. The primary uncertainty is the actual yields that might be obtained routinely under large area field conditions for different species under different geographical conditions. This can dramatically influence economics. With the possible exception of oil palm, yields for the new tree crops reported in this paper are questionable, thus necessitating experimental plantings. In this regard D. H. Scanlon (pers. commun, and ref. 31 ) of the Tennessee Valley Authority is conducting a 10-year study to accurately determine pod production yields of honeylocust. He has planted 848 grafted 'Millwood' and 'Calhoun' trees on six 0"5 to l'0 ha plots in six different states. One of us (G.W.) is participating in the study at the International Tree Crops Institute, USA, and has also put in a demonstration planting of persimmon in Kentucky. High yielding varieties need to be identified and propagated -- some progress has been made in this area. The effects of pests and diseases should be assessed. Another potential problem is that most species require 5 to 10 years to reach production, which can present cashflow problems for undercapitalized commercial ventures. Harvesting is also a
M. Seibert, G. Williams, G. Folger, T. Milne
63
potential labor- and cost-intensive requirement, but whole-tree-shakers and mechanical harvesters, currently used with some nut and fruit trees, could be adapted for use with energy-producing tree crops. Production of enough material to warrant building commercial fermentation/distillation facilities or oil-processing plants might also be a problem, but with sufficient planning this should not be insurmountable. Farm-size facilities should be investigated. CONCLUSIONS The uncertainties and the current oil glut notwithstanding, we are in a period of increasing environmental pressure to implement policies that can lead to a sustainable society. Failure to heed Nature's warning could result in collapse of civilization on a worldwide scale, as has been documented on local and regional scales many times in the p a s t y Tree-crop production for both food and fuel could be part of a worldwide strategy aimed at attaining sustainable and non-polluting supplies of resources. Finally, even if tree crops are inappropriate or uneconomic under US conditions, the conditions prevalent in the developing world may warrant application there, while systems for the developed world are being perfected. ACKNOWLEDGEMENTS The authors would like to thank J. A. Duke, P. Felker, D. H. Scanlon, H. W. Scheld, and R. J. Seibert for photographs of trees discussed in this paper and unpublished information. This work was supported by the Biomass Energy Technology Division, US Department of Energy. The genesis of this article, and the source of many of the citations given, was a workshop entitled Tree Crops f o r Energy Co-Production on Farms, held at Estes Park, Colorado, 12-14 November, 1980. The workshop was chaired by Thomas Milne and was guided by the SERI Tree Crop Task Force consisting of Michael Seibert, Robert Inman, Donald Hertzmark, and Carl Strojan. Limited copies of the proceedings are available from T. Milne at SERI. REFERENCES 1. Smith, J. R. (1950). Tree crops, a permanent agriculture, 2nd Edition, Devin-Adair Co., New York. First published by Harcourt Brace, New York (1929) and reprinted in paperback from Harper & Row, New York (1978).
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2. Brown, L. R. (1980). Food or fuel: New competition for the world's cropland, Worldwatch Paper 35, The Worldwatch Institute, Washington, DC. 3. Williams, G. (1981). Tree crops for energy production in Appalachia. In: Tree crops for energy production on farms, SERI/CP-622-1086, available from NTIS, Springfield, VA, pp. 7-20. 4. Westwood, M. N. (1978). Temperate-zone pomology, W. H. Freeman, San Francisco, pp. 228. 5. Ritter, C. M. & McKee~ G. W. (1964). The elderberry: History, classification, and culture, Pennsylvania State University Agricultural Experiment Station Bulletin 709, University Park, Pennsylvania, 22 pp. 6. Podems, M. & Bortz, B. (1975). Ornamentals for eating, Rodale Press, Emmaus, Pennsylvania, 67 pp. 7. Waino, W. W. & Forbes, E. (1941). The chemical composition of forest fruits and nuts from Pennsylvania. J. Agri. Res., 62,627-35. 8. Park, B. C. (1942). The yield and persistence of wildlife food plants. J. Wildlife Management, 6, 118-20. 9. Slate, G. L. (1955). Minor fruits. Natrl Hort. Magazine, 34 (3), 139-49. Also a letter from G. L. Slate, New York State Agric. Expt. Station to J. W. Hershey, Downingtown, PA, 16 July, 1951 (for a copy contact Gregory
Williams). 10. Detwiler, S. B. (1947). Notes on honeylocust, Washington, DC, USDA Soil Conservation Service, 197 pp. 11. Williams, G. (1980). Honeylocust, an update, North American Pomona, 13, 235-7. 12. Thomas, C. C. (1924). The Chinese jujube, USDA Bulletin No. 1215, US Government Printing Office, Washington, DC, 31 pp. 13. Felker, E, Clark, P. R., Osborn, J. E & Cannell, G. H. (1981). Utilization of mesquite (Prosopis spp.) pods for ethanol production. In: Tree crops for energy co-production on farms, SERI/CP-622-1086. Available from NTIS, Springfield, VA, pp. 65-78. 14. Chaffield, C. & McLaughlin, L. I. (1928). Proximate composition of fresh fruits, USDA Circular No. 50, Washington, DC, US Government Printing Office, 19 pp. 15. Barnhart, E. (1980). Personal communication. New Alchemy Institute, East Falmouth, Massachusetts. 16. Bargandzhiya, A. G. (1977). Biological characteristics of flowering and fruiting of oriental persimmon, Byulleten' vsesoyuznogo ordena Lenina Instituta Rastenievodstva imeni N.I. Vavilova, No. 68, 36-38. In Russian. 17. Claypool, J. V. (1980). Personal communication, St. Elmo, Illinois. 18. Jaynes, R. A. (ed.) (1979). Nut tree culture in North America, Northern Nut Growers Association, Hamden, Connecticut, 466 pp. 19. Beaverlodge Nursery (1979). Hardy plants for Northern gardens, Beaverlodge, Alberta, 45 pp. 20. Smock, R. N. & Neubert, A. M. (1950). Apples and apple products, Interscience Publishers, New York.
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21. Jacobs, P. B. & Newton, H. E (1938). Motor fuels from farm products, USDA Miscellaneous Publication No. 327, US Government Printing Office, Washington, DC. 22. Shallenberger, R. S. (1974). Occurrence of various sugars in foods. In: Sugars in nutrition, H. L. Sipple and K. W. McNutt (eds), Academic Press, New York, pp. 67-80. 23. Woodroof, J. G. (1979). Tree nuts: production, processing, products, 2nd edition, AVI Publishing, Westport, Connecticut, 731 pp. 24. Payne, J. A. (1978). Chinese chestnut production in the Southeastern United States: Practice, problems, and possible solutions. In: Proceedings of the American chestnut symposium, West Virginia University Books, Morgantown, West Virginia, pp. 20-4. 25. Agricultural Research Service Northeastern Region (1976). Growing black walnuts for home use, U S D A Leaflet No. 525, US Government Printing Office, Washington, DC, 9 pp. 26. Halls, L. K. (ed.) (1977). Southern fruit-producing woody plants used by wildlife, USDA Forest Service General Technical Report SO-16, US Government Printing Office, Washington, DC, 235 pp. 27. Langworthy, C. E & Holmes, A. D. (1917). The American papaw and its food value. J. Home Econ., 9, 505-11. 28. Scanlon, D. H. (1981). A case study of honeylocust in the Tennessee Valley region. In: Tree crops for energy co-production on farms, SERI/CP-6221086, available from NTIS, Springfield, VA, pp. 21-31. 29. Williams, G. & Merwin, M. L. (1983). Energy- and soil-conserving perennial crops for marginal land in temperate climate. In: Environmentally sound agriculture, W. Lockeretz (ed.), Praeger Publishers, New York, pp. 317-32. 30. US Forest Service (1965). Silvics of forest trees of the United States, Agriculture Handbook No. 271, USDA Forest Service, pp. 187-201. 31. Tennessee Valley Authority (1984). TVA biomass fuels update II, TVA OACD-84/3, available from NTIS, Springfield, VA, pp. 13-14. 32. McDaniel, J. C. ( 1981 ). A plant breeder looks at some American tree crops: Morus, Gleditsia, and Diospyros. In: Tree crops for energy co-production on farms, SERI/CP-622-1086, available from NTIS, Springfield, VA, pp. 113-18. 33. Smith, J. L. & Perino, J. V. ( 1981 ). Osage orange (Maclura pomifera) history and economic uses. Economic Botany, 35 ( 1 ), 24-4. 34. Clopton, J. R. & Roberts, A. (1949). Osage orange -- A potential source of edible oil and other industrial raw materials. J. Oil Chem. Soc., 26,470-2. 35. Hokes, J., Chism, G. & Hansen, E M. T. (1976). Osage orange -- A source of proteolytic enzyme. Ohio Report on Research and Development, 61 ( 1 ), 11-13. 36. Leakey, C. L. A. (1977). Breadfruit reconaissance study in the Caribbean region, Monograph of CIAT/Inter-American Development Bank. 37. Avgerinos, G. C. & Wang, D. I. C. (1981). Utilization of mesquite and
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honeylocust pods as feedstocks for energy production. In: Tree crops for energy co-production on farms, SERI/CP-622-1086, available from NTIS, Springfield, VA, pp. 209-17. Ogden, R. L. (1983). Attempt to ferment sugars in locust tree bean. Agroforestry Review, 3 (3 and 4), 5. Kaufman, K. R. (1984). Testing of vegetable oils in diesel engines. In: Fuels and chemicals from oilseeds. Technology and policy options, E. B. Schultz, Jr. and R. P. Morgan (eds), AAAS Symposium 91, Westview Press, Boulder, CO, pp. 143-75. Weisz, P. B., Haag, W. O. & Rodewald, G. (1979). Catalytic production of high-grade fuel (gasoline) from biomass compounds by shape-selective catalysis. Science, 206, 57-8. Hodge, W. H. (1975). Oil-producing palms of the w o r l d - A review. Principes, 19 (4), 119-36. Emergy, W. L. (ed.) (1984). 1984 Commodity Year Book, Commodity Research Bureau, Jersey City, NJ. Scheld, H. W., Cowles, J. R., Engler, C. R., Kleiman, R. & Schultz, Jr., E. G. (1984). Seeds of the Chinese tallow tree as a source of chemicals and fuels. In: Fuels and chemicals from oilseeds. Technology and policy options, E. B. Schultz, Jr., and R. P. Morgan (eds), AAAS Symposium 91. Westview Press, Boulder, CO, pp. 81-107. FAO (1984). 1983 FAO Production Yearbook, Vol. 37, FAO of the UN, Rome, pp. 307. Cowles, J. R. & Scheld, H. W. (1984). Cultural and management practices for the Chinese tallow tree as a biomass fuel source, Final DOE Report, Contract No. ET-78-G-01-3037. Scheld, H. W., Bell, N. B., Cameron, G.N., Cowles, J. R., Engler, C. R., Krikorian, A. D. & Schultz, Jr, E. B. (1981). The Chinese tallow tree as a cash and petroleum-substitute crop. In: Tree crops for energy coproduction on farms, SERI/CP-622-1086, available from NTIS, Springfield, VA, pp. 97-111. Opeke, L. W. (1982). Tree crops, John Wiley and Sons, New York, pp. 251-74. Duke, J. A. (1977). Palms as energy sources: A solicitation. Principes, 21, 60-2. Freedman, D. (1981). Preliminary analysis of the potential for ethanol production from honeylocust pods. In: Tree crops for energy co-production on farms, SERI/CP-622-1086, available from NTIS, Springfield, VA, pp. 121-34. Brown, L. R. (1982). R-and-D for a sustainable society. Am. Scientist, 70 (1), 14-17.